As for a method for forming a film on a substrate, e.g., a semiconductor wafer (hereinafter, referred to as “wafer”), there are known a so-called ALD (Atomic Layer Deposition) method for sequentially supplying a plurality of reactant gases that react with one another to a wafer, and a MLD (Multi Layer Deposition) method (hereinafter, both will be referred to as “ALD method”).
There are suggested various gas supply units for supplying reactant gases to a wafer in such a film forming method. For example, Japanese Patent Application Publication Nos. 2002-327274 and 2006-299294 disclose a shower head including vertically arranged gas diffusion spaces (referred to as “spaces 11a and 11b” in JP2002-327274A and “a gas diffusion space 50 and a space 81” in JP2006-299294A) partitioned by an intermediate plate among a plurality of plates provided at multiple stages spaced apart from each other in a vertical direction, and a plurality of gas channels extending from the respective diffusion spaces to a bottom surface of a lowermost shower plate.
In such a shower head, reactant gases are respectively supplied from the gas diffusion spaces separated from each other. Therefore, the reactant gases are not mixed with each other in the gas diffusion spaces, thereby preventing deposition of reaction products to the shower head.
In order to supply the reactant gases from the vertically arranged gas diffusion spaces without being mixed with each other, it is required to provide a plurality of lines for gas channels which penetrate through the lower gas diffusion space to communicate with the upper gas diffusion space. This makes the structure of the shower head complicated.
Therefore, there has been developed a simple shower head for selectively supplying a plurality of reactant gases into a common gas diffusion space. In the case of using the common gas diffusion space, gas replacement needs to be performed by supplying an inert gas or the like between supply of a reactant gas and supply of a next reactant gas in order to prevent deposition of reaction products.
When the reactant gases are replaced, time required for the replacement operation needs to be minimized in order to efficiently perform film formation. Recently, a deposited film of nanometer order may require film thickness uniformity (e.g., 10% to be described later (percentage of standard deviation σ divided by an average value)) of 2% or less over the wafer surface. Therefore, it is required to develop a shower head capable of realizing film formation with excellent in-plane uniformity as well as effective replacement.
In response to such demands, the shower head disclosed in Patent Documents 1 and 2 has a large gas diffusion space extending over a region corresponding to the entire surface of the wafer. Even if reactant gases and a replacement gas are selectively supplied to one side of the gas diffusion space, a long period of time is required for the replacement operation.
Further, JP2002-327274A and JP2006-299294A disclose gas supply units for supplying reactant gases into the gas diffusion spaces (gas injection openings 121 formed at a pipe portion 10j in JP2002-327274A, and a gas injection port 56 having a gas injection opening 55 and a gas injection line 83 in JP2006-299294A). However, in the shower head for selectively supplying the reactant gases and the replacement gas, there is no description on technical features of the gas supply units which are required to improve uniformity of a film to be formed.
Therefore, there has been developed a film forming apparatus disclosed in Japanese Patent Application Publication No. 2009-224775 which increases efficiency of replacement by providing a shower head (referred to as “gas supply nozzle” in JP2009-224775A) smaller than a wafer as a film formation target at a central area of a ceiling portion having an inclined surface that becomes gradually wider from a central side toward a peripheral side.
However, a flow speed of a reactant gas injected from a gas supply opening formed immediately below a gas supply line for introducing a gas into the shower head among a plurality of gas supply openings formed at the shower head may be increased compared to that of a reactant gas injected from a gas supply opening formed at a position separated from the position immediately below the gas supply line. As a result, a difference in flow speeds of the gases injected from the gas supply openings leads to a difference in the amount of reactant gases adsorbed onto the wafer and the film thickness may be slightly varied in the surface of the wafer. However, if high in-plane uniformity (1σ%) of about 2% or less is required as described above, even a slight difference in the film thickness needs to be improved.